var
 n,q,i,j,k,ans,temp,s,t,l,len:longint;
 a:array[0..150]of longint;
 f,w:array[0..150,0..150]of longint;

 function min(a,b:longint):longint;
  begin if a<b then exit(a) else exit(b);end;

begin
assign(input,'linka.in');reset(input);
assign(output,'linka.out');rewrite(output);
 readln(n,q);
 j:=0;
 for i:=1 to q do read(a[i]);

 for i:=1 to q-1 do
  for j:=1 to q-i do
   if a[j]>a[j+1] then
    begin temp:=a[j];a[j]:=a[j+1];a[j+1]:=temp;end;

 ans:=0;
 if a[1]=1 then begin ans:=ans+a[2]-2;s:=1;a[1]:=0;end
  else begin s:=0;a[0]:=0;end;
 if a[q]=n then begin ans:=ans+n-1-a[q-1];t:=q;a[q]:=n+1;end
  else begin t:=q+1;a[q+1]:=n+1;end;

 fillchar(f,sizeof(f),$3f);
 len:=t-s;
 for i:=s to t-1 do f[i,i+1]:=0;
 for l:=2 to len do
  for i:=s to t-l do
   begin
    j:=i+l;
    for k:=i+1 to j-1 do
     f[i,j]:=min(f[i,j],f[i,k]+f[k,j]+a[j]-a[i]-2);
   end;

 writeln(ans+f[s,t]);
close(input);close(output);
end.